The world's most famous hominin fossil is Lucy, an Australopithecus afarensis that had a chimp-like brain but walked upright like a human. Now a new discovery reveals that Lucy actually shared her world with another, very different hominin species.

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Both Lucy and this newly discovered fossil date back between 3.2 and 3.4 million years ago, and they both lived in the Afar region of what is now Ethiopia. Unlike Lucy, whose foot is essentially like that of humans, the toes of this new foot fossil, known as the Burtele partial foot, are clearly adapted to climb trees like a chimpanzee. It seems that, while Lucy and Australopithecus afarensis ruled the grounds of the Afar region, the trees belonged to the Burtele species.

The idea of two hominin species living in close quarters with each other isn't unknown. After all, we know humans and Neanderthals got close enough to interbreed — which, just in case it wasn't already 100% obvious, is very close indeed — but those were two essentially similar species that only really coexisted for about ten thousand years or so. This new discovery puts two fundamentally different kinds of hominin — one bipedal, the other arboreal — in the same region, potentially for hundreds of thousands of years. That's a pretty remarkable thought.

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For a good overview of just what this find means, you can check out the video up top from Nature. Team member Dr. Bruce Latimer of Case Western discusses the find:

"This discovery was quite shocking. These fossil elements represent bones we've never seen before. While the grasping big toe could move from side to side, there was no expansion on top of the joint that would allow for expanded range of movement required for pushing off the ground for upright walking. This individual would have likely had a somewhat awkward gait when on the ground.

"It is now clear that the adaptation to terrestrial bipedality was not a single, isolated event. Rather, one group (Lucy's species) totally relinquished the arboreal habitat and became functionally committed, long-distance ground walkers while another group, represented by the Burtele foot, maintained a climbing foot and stayed, at least part of the time, in the trees. It is now apparent which group succeeded."

So what exactly makes Lucy and the Burtele foot different? It's all about the morphology of the big toe. There are a lot of anatomical shifts that need to take place for a species to successfully move about on only two legs, including our larger heels and the arches of the foot, both of which help us remain stable while walking. Another key adaptation is the realignment of the big toe so that it points in the same direction as the other four toes. That's how Lucy's big toe pointed, but the Burtele foot still has an opposable big toe that would have allowed its feet to grasp tree branches for a surer footing. Writing about the discovery in Nature, Harvard evolutionary biologist Daniel Lieberman, who was not involved in the research, goes into great detail about what made the Burtele foot so distinctive:

The fossil...comprises eight bones, all from the front half of a single right foot. In many ways, the foot is ape-like, especially resembling that of a gorilla. The big toe is short, very divergent, and apparently capable of grasping against the second toe. In addition, the toe bones are generally long and slightly curved, placing them between those of apes and hominins, although the fourth metatarsal bone is curiously long, like a monkey's.

What's particularly interesting about the foot, according to Lieberman, is how it still shows a few traces of bipedalism. The metatarsal and phalange bones in the other four, non-big toes have features that resemble those of later hominins, although they are occasionally seen in chimpanzees and gorillas as well. The Burtele species wold have been able to hyperextend its toes to push off the ground, which would be useful in bipedal motion, and it shows some subtle signs of, if not a fully formed arch, then at least something vaguely like it. This wasn't a fully arboreal creature, though admittedly it was pretty close to it.

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The Burtele foot hasn't yet been classified in a particular species, but it likely has something to do with Ardepithecus ramidus, a 4.4 million year old fossil that was primarily built for living in the trees but, like Burtele, also had some tentative adaptations towards bipedalism. Exactly what Ardipithecus is in the larger context of human evolution remains somewhat controversial, but it is often considered one of the most ancient hominin genera and the one particularly closely related to our common ancestor with chimpanees.

Whether or not the Burtele foot represents a much later example of the Ardipithecus genus, it definitely reveals that hominin species used vastly different ways to get around — both in the trees and on the ground — for far longer than we previously thought. It also proves that there was definitely more than one pre-human species present at any one time during the Pleistocene - between 3 and 4 million years ago - which had not been incontrovertibly demonstrated until now.

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This find is one of the starkest demonstrations yet that evolution is a complex, messy phenomenon, full of alternative paths and divergences, only some of which are ultimately successful. Our evolutionary family tree has plenty of distant relations that went in totally different paths than we did, and we're only likely to discover even more of them as we excavate further.